#!/tmp/bleadperl/bin/perl -w 

use re 'eval';

my $re;
my $len = 20;

$_ = "this is a long string, and I want to insert newlines every 20 chars";

$re = qr{
  (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' })
  (?(?{ ($+[0] - $-[0]) < $len })
    (??{ '\W{0,' . ($len - ($+[0] - $-[0])) . '}' })
    (?(?{ ($+[0] - $-[0]) < $len })
      (?(?=
        (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' }))
        (??{ $re })
      )
    )
  )
}x;

s{($re)}{$1\n}xg;

print;

##</code><code>##

$re = qr{
  (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' })
  (?(?{ ($+[0] - $-[0]) < $len })
    (??{ '\W{0,' . ($len - ($+[0] - $-[0])) . '}' })
  )
  (?(?{ ($+[0] - $-[0]) == $len })
    (?&done)
  )
}x;

s{($re+)(?%done)}{$1\n}xg;

##</code><code>##

$re = qr{
  (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' })
  (?(?{ ($+[0] - $-[0]) < $len })
    (??{ '\W{0,' . ($len - ($+[0] - $-[0])) . '}' })
  )
  (?(?{ ($+[0] - $-[0]) == $len }) (?;) )
}x;

s{($re+)}{$1\n}xg;

##</code><code>##

while (1)
  do_pattern($re);
  if ($seen_qu_semi) { last }
}

##</code><code>##

$re = qr{
  (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' })
  (?(?{ ($+[0] - $-[0]) < $len })
    (??{ '\W{0,' . ($len - ($+[0] - $-[0])) . '}' })
  )
}x;

s{
  (
    (?:
      (?(?{ ($+[0] - $-[0]) == $len })(?!))
      $re
    )+
  )
}{$1\n}xg;